This invention relates to automatic lens focusing mechanisms for photographic cameras and more particularly, it concerns an improved photoelectric pulse generating unit for monitoring the focus position of a camera objective lens driven in response to a range detection system.
In a commonly assigned co-pending application Ser. No. 729,289, filed Oct. 4, 1976, in the name of Edwin K. Shenk, and now abandoned, there is disclosed a camera lens focusing mechanism in which an electronic logic circuit responds to a range signal related directly to the camera-subject distance in a manner to produce a train of pulses, the number of which is representative of the lens mount axial position at which the subject will be in focus. Such pulses are gated into a counter and used for operating a drive motor coupled mechanically to the lens mount. The lens mount is coupled rotatably with an encoder wheel, operative as a component of an auxiliary pulse generator in a feed-back systen, so that operation of the lens drive motor causes the auxiliary pulse generator to produce a predetermined number of pulses for each axial unit displacement of the lens mount. The logic circuit responds to the output of the auxiliary pulse generator to determine when the lens mount has been moved to the position determined by the pulse train counter to be proper for focusing the subject to be photographed. Such logic circuit response results in engagement of a pawl with a lens coupled rack to stop movement of the lens mount at the proper focusing position even though the drive motor may continue to rotate. A slip clutch in the coupling between the motor and the lens mount facilitates this latter operating characteristic.
In the feed-back system disclosed in the aforementioned copending application, the pulses corresponding to lens displacement from a pre-established position are preferably generated by rotation of slots or apertures in the lens encoder wheel between a light source, such as a light emitting diode (LED) and a photocell. The increments of angular spacing between the slots or apertures in the encoder wheel are directly related to increments of lens mount rotation which, because of a pre-established thread or camming pitch of the lens mount, represent axial increments of lens focusing movement. This arrangement has a distinct advantage from the standpoint of elimination of frictional or mechanical drag on movement of the lens mount to appropriate focusing positions and as well, it is easily adaptable to electronic circuitry suited for automatic focusing systems.
There are, however, problems presented from the standpoint of the size of the photo-electric feed-back system capable of being accomodated in a viable automatic focusing camera design, facility for assembly in the shutter housing of a camera, calibration of photocell generated pulses with lens mount movement from a given reference position, and elimination of erratic pulse generation due to exposure of the photocell to stray light.